Conventionally, a range image producing apparatus has been proposed by which while pulse light is irradiated, reflection light from measurement objects, which are located within previously set measuring ranges, is received by photoelectric sensors having a plurality of pixels, and then, range images are produced from output data of the photoelectric sensors (refer to, for instance, US Patent Application Publication No. 2004-233416).
It should be understood that a range image corresponds to such an image that a distance up to an imaged object is represented by gradation (pixel values) of an image. Since such a range image is employed, distances up to a plurality of objects located within a wide range can be measured within one time.
In the range image producing apparatus, this apparatus is arranged as follows: That is, as shown in FIG. 11, while pulse light is irradiated, after a predetermined delay time “Tv (=n×ΔTv, n=0, 1, 2, 3, . . . )” has elapsed from this pulse irradiation timing, such an operation that an integration window is opened (namely, light is exposed) only for such a time period equal to a pulse width of the irradiated pulse light so as to receive reflection light is repeatedly carried out a large number of times, while this delay time Tv is shifted by ΔTv. Here, one subject point is considered, D is constant, and To is constant. Then, such a delay time Tv that a received light amount (integrated voltage U) becomes maximum is detected as a propagation time “To,” which is required to reciprocate the pulse light over a distance up to a measurement object which reflects thereon the irradiated pulse light. Then, a distance “d (=c×To/2)” measured up to the measurement object is calculated based upon this propagation time “To” and a velocity “c” of light.
In the conventional range image producing apparatus, for example, in such a case that distance resolution is selected to be several tens of centimeters to several meters, and a measuring range is selected to be approximately 100 meters, the change width ΔTv of the delay becomes on the order of 0.1 nanoseconds to 1 nanosecond, and a repetition time of measurements becomes on the order of several hundreds to several thousands times.
In other words, the conventional range image producing apparatus owns the following problems. That is, the very high precision control circuit is required so as to control the integration window within approximately 0.1 nanoseconds, and further, the large-scaled control circuit which processes a very large amount of measurement results is necessarily required, resulting in a large-scaled apparatus.
Moreover, in such a case that the change width ΔTv of the delay time Tv is decreased so as to improve the distance resolution, and/or the measuring range is widened so as to achieve higher performance, there is another problem that a total repetition number of measurements is further increased.
Also, the conventional range image producing apparatus has another problem. That is, since the irradiating operation of the pulse light must be repeated many times during a single measuring operation, this conventional apparatus is not suitably applied to such a requirement of a high real-time characteristic of the apparatus. Also, when an object to be measured is moved, a distance between this conventional apparatus and the object is changed every time the pulse light is irradiated, so that precision of measurement results is lowered.